This invention is related to organic electroluminescent (EL) devices, and more specifically, to organic EL devices with excellent performance characteristics. Organic EL devices are desired that are capable of providing uniform luminescence, saturated color in blue, green and red, and low driving voltages. The organic EL devices of the present invention enable in embodiments the aforementioned characteristics, and which devices contain charge transport/luminescent materials comprised of a new class of carbazole compounds, and wherein these devices can be selected for use in flat-panel emissive display technologies, including TV screens, computer screens, and the like.
An organic EL device can be comprised of a layer of an organic luminescent material conductively sandwiched between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. Several prior art organic EL devices have been constructed from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene. However, these devices usually require excitation voltages on the order of 100 volts or greater.
Organic EL devices with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Another alternate device configuration is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode. Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent dopant material.
In U.S. Pat. No. 4,539,507, the disclosure of which is totally incorporated herein by reference, there is disclosed an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p-tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4xe2x80x2-bis(5,7-di-tert-pentyl-2-benzoxyzolyl)stilben, and an indium cathode. In U.S. Pat. No. 6,229,012, the disclosure of which is totally incorporated herein by reference, there are illustrated devices with certain carbazoles.
U.S. Pat. No. 4,720,432, the disclosure of which is totally incorporated herein by reference, discloses an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport.
U.S. Pat. No. 4,769,292, the disclosure of which is totally incorporated herein by reference, discloses an EL device employing a luminescent zone comprised of an organic host material capable of sustaining hole-electron recombination and a fluorescent dye material capable of emitting light in response to energy released by hole-electron recombination. One host material disclosed in the ""292 patent is an aluminum complex of 8-hydroxyquinoline, and more specifically, tris(8-hydroxyquinolinate)aluminum.
U.S. Pat. No. 5,409,783, the disclosure of which is totally incorporated herein by reference, discloses a red-emitting organic EL device containing a dopant of a tris(8-hydroxyquinolinate)aluminum with a red fluorescent dye. Further, blue-emitting organic EL devices are illustrated in, for example, U.S. Pat. Nos. 5,151,629 and 5,516,577, the disclosures of which are totally incorporated herein by reference.
While progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of a number of devices are still below expectations in several instances. Further, for visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers for blue, green and red. Moreover, although the use of aromatic tertiary amines as hole transport materials in organic EL devices is known, the amine compounds selected, such as N,N,Nxe2x80x2,Nxe2x80x2-tetraarylbenzidines, have a tendency to form complexes with the EL electron transport materials in contact therewith, thus resulting in, for example, emission with a broad spectra. This complexation for blue emitting devices results in the electron transport materials retaining a larger band gap than those used in devices with green or red emission. Thus, there continues to be a need for hole transport compositions for organic EL devices, and which materials are suitable for selection in blue emitting devices. Also, there is a need for EL hole transports which are vacuum evaporable and form films with excellent thermal stability. There is also a need for luminescent compositions which are capable of providing uniform and satisfactory emission in the visible spectrum from blue to red colors. In particular, there is a need for efficient blue luminescent materials for organic EL devices, which can be doped with a fluorescent dye to provide different colors by a downhill energy transfer process. Further there is also a need for luminescent compositions which can enhance the EL charge transporting characteristics thus lowing device driving voltages. Therefore, one feature disclosed herein is to provide charge transport/luminescent materials comprised of a new class of carbazole compounds and wherein there is avoided or there is minimized poor film forming properties, thermal instability, and weaker fluorescent properties.